1. Field of the Invention
The present invention is related to a parallel redundant power system and the control method for the same, and more particularly to a power system that utilizes an unbalanced power calculation and a cross current limitation technique to control the response of the inverter.
2. Description of Related Arts
With the almost total dependence of modern societies on electricity for employment, living, communication and entertainment, any disruption to an electricity supply brings about a temporary halt to civilization as we know it. Especially in the present age where a breathtaking variety of electronic products are widely used, serious economic loss may possibly occur if a company loses critical data when the electricity supply is suddenly interrupted.
To solve the above problem, the uninterruptible power supply (UPS) has been developed for providing a stable power supply. The most important objective of the UPS is to continuously supply stable power to the load even when the input line voltage experiences unusual and damaging situations. Therefore, the reliability of the UPS is very important when the UPS is adopted as the protective power supply source. In the actual environment, the UPS may face many difficult problems, for example the fluctuation of the input voltage, thunderstorm interference, the sudden change of the load (the load may even short), and the continuous operation for twenty-four hours a day. For the common on-line UPS, the reliability may reach 99.9% while the manufacturing cost is in the acceptable range for the fabricators. However, if the manufacturer intends to improve the reliability to be higher, not only will the circuit become more complex, but also the cost will also be greatly increased. Therefore, under the premise that there should not be much increase in the cost, designers have utilized a single UPS to accomplish a redundant power supply system to improve the reliability, as disclosed in U.S. Pat. No. 5,257,180. It should be noted that the cited patent is directed to inverters, not to a UPS.
There is no doubt that the reliability of these inverters will be improved by implementation, however, most of the control means for the power supply system is accomplished by hardware, so the gain of the controllers is unable to be modified and thus the transient (dynamic) response of the power supply system is difficult to handle. The analyses of the cross current and the parallel operation disclosed by the cited patent utilizes a virtual vector, wherein the virtual vector may even be further divided into two components (vectors). However, the amplitude difference and the phase difference among the inverters caused by the difference of the inverter impedances is not of concern here. The method taught by the cited patent is so complex that a high reliability may not be obtained and the fabrication cost is increased to a non-commercially.
Furthermore, if the inverter is used as an adaptive inverter for the UPS, the cited patent does not take into account the input DC voltage of the inverter from the bus, and the influence generated while the inverter is coupled to any other module is also not considered either. Moreover, the synchronizing signal of the patent is supplied from external circuits. Once the synchronize signal experiences failure, the entire power system will breakdown.